1. Field of the Invention
The present invention relates to a method of and an apparatus for inspecting a paint coating applied on a workpiece in the form of a coated thin film.
2. Description of the Prior Art
During the assemblage of an automobile, and prior to the installation of a windowshield and a rear windowpane on respective window frames in the automobile, a sealing material is generally lined to a peripheral region of each windowpane. It is also a customary practice to apply a black-colored primer direct to the peripheral region of each windowpane, prior to the application of the sealing material, to form a primer undercoat for the purpose of enhancing the reliability in bonding of the subsequently formed sealant coating to the windowpane.
The Japanese Laid-open Patent Publication No. 61-242666, published Oct. 28, 1986, and applied for patent in Japan by the same assignee of the present invention, discloses an inspecting apparatus for inspecting the coating condition of the primer undercoat on the windowpane. According to this prior art inspecting apparatus, a light emitter and a light receiver are positioned on respective sides of the windowpane so that the amount of light passing through the primer-undercoated peripheral region of the windowpane, that is, the light transmissivity, can be detected by the light receiver. A light signal emerging from the light receiver and indicative of the detected light transmissivity is supplied to an amplifier where the light signal is amplified. The amplified light signal emerging from the amplifier is compared with a predetermined level by a comparator which subsequently generates a control signal according to the difference between the amplified light signal and the predetermined level.
Where the windowpane to be inspected is of a type having its peripheral region lined with black-colored ceramics to provide a so-called ceramics-lined glass, the light transmissivity as measured across the ceramics-lined peripheral region of the windowpane is extremely low, for example, lower than about 0.1%, because of the presence of the black layer of the primer undercoat and, moreover, the difference in light transmissivity between the ceramics-lined windowpane with the primer undercoat having not yet been formed thereon and the ceramics-lined windowpane with the primer undercoat having been formed is very small.
In view of the foregoing, the prior art inspecting apparatus of the type discussed hereinabove has been found ineffective to provide an accurate measurement not only because the light signal outputted from the light receiver is very low, but also because the amount of change in level of the light signals which are generated from the light receiver when the windowpane having no primer undercoat formed thereon is inspected and when the windowpane having the primer undercoat formed thereon is inspected, respectively, is very small.
In other words, while the peripheral region of the windowpane has been lined with the black-colored ceramics, the addition of the black-colored primer coating to the same peripheral region of the windowpane considerably reduces the light transmissivity of the windowpane to a value lower than that exhibited when the primer coating has not yet been formed on the windowpane. Therefore, with this type of windowpane, neither an automatic inspection of the coating condition with the use of the previously discussed prior art inspecting apparatus or an industrial television monitor system nor an inspection based on the naked eyes of a skilled inspector is possible.